Firefox x64 build "unknown processor architecture"

[Nick]

I've been trying to compile a 64-bit version of Firefox on my machine,
but I've run into a problem that I can't figure out. All of the
instructions I've been able to find via Google about compiling for x64
are from like 2005, so I would think that 4 years later this would be
at least pretty commonplace...

Anyway, when I compile I get this error:

c:\users\nick\proj\192src\objdir\dist\include\prcpucfg.h(301) : fatal
error C118
9: #error : unknown processor architecture
make[4]: *** [jsapi.obj] Error 2
make[4]: Leaving directory `/c/Users/Nick/proj/192src/objdir/js/src'
make[3]: *** [libs_tier_js] Error 2
make[3]: Leaving directory `/c/Users/Nick/proj/192src/objdir'
make[2]: *** [tier_js] Error 2
make[2]: Leaving directory `/c/Users/Nick/proj/192src/objdir'
make[1]: *** [default] Error 2
make[1]: Leaving directory `/c/Users/Nick/proj/192src/objdir'
make: *** [build] Error 2

I found a bug report about this, and the last post (from Jan 2008)
says that this problem has been fixed:
https://bugzilla.mozilla.org/show_bug.cgi?id=225859

However, I'm still getting the same problem... This is what my
mozconfig file looks like:

. $topsrcdir/browser/config/mozconfig
mk_add_options USE_64=1
mk_add_options MOZ_CO_PROJECT=browser
mk_add_options MOZ_OBJDIR=@TOPSRCDIR@/objdir
ac_add_options --enable-64-bit
ac_add_options --target=x86_64-pc-mingw32
ac_add_options --host=x86_64-pc-mingw32
ac_add_options --disable-ogg
ac_add_options --with-windows-version=600

I've been trying to figure this out for hours, so any input would be
great. Thanks!

[ToastyJustice]

I've been getting this same build error trying to perform a 64-bit
build of firefox, and I also cannot find any more documentation to
help. I've been combing the build docs, forums, and some of the news/
mailing list as well. If anyone finds a solution for this it would be
greatly appreciated.